Electrolyte for photoelectric conversion elements, and photoelectric conversion element and dye-sensitized solar cell using the electrolyte

ABSTRACT

An object of the present invention is to provide an electrolyte for photoelectric conversion elements, and a photoelectric conversion element and a dye-sensitized solar cell using the electrolyte, wherein high energy conversion efficiency can be achieved while substantially not including iodine. The electrolyte for a photoelectric conversion element of the present invention includes an ionic liquid (A) and a carbon material (B). The carbon material (B) is a carbon material (B1) displaying a pH, measured by a pH measuring method specified in Japanese Industry Standard (JIS) Z8802, of from 2 to 6 and/or a boron-modified acetylene black (B2). A content of the carbon material (B) is from 10 to 50 parts by mass per 100 parts by mass of the ionic liquid (A).

TECHNICAL FIELD

The present invention relates to an electrolyte for photoelectricconversion elements, and a photoelectric conversion element and adye-sensitized solar cell using the electrolyte.

BACKGROUND ART

In recent years, environmental issues such as global warming and thelike that are attributed to increases in carbon dioxide have becomeserious. As a result, non-silicon solar cells have gained attention assolar cells that have little environmental impact and that also allowfor reduced manufacturing costs; and research and development of such ismoving forward.

Among non-silicon solar cells, the dye-sensitized solar cell developedby Graetzel et al. in Switzerland has attracted attention as a new typeof solar cell. As a solar cell using organic materials, these solarcells have advantages such as high photoelectric conversion efficiencyand lower manufacturing costs than silicon solar cells.

However, dye-sensitized solar cells are electrochemical cells, andtherefore use organic electrolytic solutions and/or ionic liquids aselectrolytes. In cases where organic electrolytic solutions are used,there is a problem in that electrical efficiency decreases due tovolitization and depletion during long-term use. Additionally, in caseswhere ionic liquids are used, while volitization and depletion thatoccur during long-term use can be prevented, there are durabilityproblems such as structural degradation caused by liquid leakage.

Therefore, research is being conducted regarding converting theelectrolyte from a liquid to a gel or solid for the purpose ofpreventing the volitization and liquid leakage of the electrolyticsolution and ensuring the long-term stability and durability of thesolar cell.

For example, Patent Document 1 describes an electrolyte compositionincluding an ionic liquid and conductive particles as main components,wherein the electrolyte composition is made into a gel (Claims 1 and 2).

Additionally, Patent Document 2 describes a dye-sensitized photoelectricconversion element having a porous photoelectrode layer made fromdye-sensitized semiconductor particles, a charge transport layer, and acounterelectrode layer in this order. The charge transport layer is madefrom a solid mixture including from 1 to 50 mass % of a p-typeconductive polymer, from 5 to 50 mass % of a carbon material, and from20 to 85 mass % of an ionic liquid (Claim 1).

However, in cases where the electrolyte composition described in PatentDocument 1 is used, when a redox couple (particularly iodine) is used inorder to achieve high energy conversion efficiency, there have beenproblems such as the metal wiring (collecting electrode), seal material,and the like that constitute the photoelectric conversion element beingcorroded due to the corrosive properties of the iodine; and thestability of the electrolyte being affected due to the volatility of theiodine.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: WO/2005/006482-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 2007-227087A

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

Additionally, as a result of investigation into the dye-sensitizedphotoelectric conversion element described in Patent Document 2, theinventors discovered that energy conversion efficiency is notsufficient.

Specifically, it is thought that when using a mixture of a p-typeconducting polymer (e.g. polyaniline, polypyrrole, and the like), acarbon material (e.g. acetylene black, and the like), and an ionicliquid as a charge transport layer, the ability of the carbon material(particularly acetylene black) itself to retain the ionic liquid(retention capacity) is low and, moreover, said retention capacity isfurther decreased as a result of mixing with the p-type conductingpolymer.

Therefore, an object of the present invention is to provide anelectrolyte for a photoelectric conversion element, and a photoelectricconversion element and a dye-sensitized solar cell using theelectrolyte, wherein high energy conversion efficiency can be achievedwhile substantially not including iodine.

Means to Solve the Problem

As a result of diligent research, the present inventors discovered thatan electrolyte for a photoelectric conversion element including an ionicliquid and a carbon material having a specific pH value at a specificratio can achieve high energy conversion efficiency while substantiallynot including iodine, and thus arrived at the present invention.

Specifically, the present invention provides the following (a) to (k).

(a) An electrolyte for a photoelectric conversion element including anionic liquid (A) and a carbon material (B), wherein

the carbon material (B) is a carbon material (B1) displaying a pH,measured by a pH measuring method specified in Japanese IndustryStandard (JIS) Z8802, of from 2 to 6 and/or a boron-modified acetyleneblack (B2), and

a content of the carbon material (B) is from 10 to 50 parts by mass per100 parts by mass of the ionic liquid (A).

(b) The electrolyte for a photoelectric conversion element described in(a), wherein a primary average particle size of the carbon material (B1)is from 0.010 to 0.050 μm.

(c) The electrolyte for a photoelectric conversion element according to(a) or (b), wherein the ionic liquid (A) includes a cation that isexpressed by the following Formula (1) or (2):

In Formula (1), R¹ is a hydrocarbon group having from 1 to 20 carbonsthat may include a hetero atom, and may include a substituent having 1to 20 carbons that may include a hetero atom. R² and R³ are eachindependently a hydrogen atom or a hydrocarbon group having from 1 to 20carbon atoms, and may include a hetero atom. However, the R³ moiety isabsent if the nitrogen atom includes a double bond. In formula (2), Q isa nitrogen, oxygen, phosphorus, or sulfur atom; and R⁴, R⁵, R⁶, and R⁷are each independently a hydrogen atom or a hydrocarbon group having 1to 8 carbons that may include a heteroatom. However, the R⁷ moiety isabsent if Q is an oxygen or a sulfur atom.

(d) The electrolyte for a photoelectric conversion element described in(c), wherein the ionic liquid (A) includes iodine ions as anions.

(e) The electrolyte for a photoelectric conversion element described inany of (a) to (d), further including less than 10 parts by mass of acarbon material (C1) having a specific surface area of from 1,000 to3,500 m²/g per 100 parts by mass of the ionic liquid (A) as anothercarbon material (C), aside from the carbon material (B).

(f) The electrolyte for a photoelectric conversion element described inany of (a) to (e), further including less than 10 parts by mass of acarbon black (C2) having a nitrogen adsorption specific surface area notless than 90 m²/g per 100 parts by mass of the ionic liquid (A) as theother carbon material (C), aside from the carbon material (B).

(g) The electrolyte for a photoelectric conversion element described in(e) or (f), wherein a total content of the carbon material (B) and theother carbon material (C) is from 10 to 50 parts by mass per 100 partsby mass of the ionic liquid (A).

(h) The electrolyte for a photoelectric conversion element described inany of (e) to (g), wherein a ratio [carbon material (B)/other carbonmaterial (C)] of the carbon material (B) to the other carbon material(C) is from 99.9/0.1 to 60/40.

(i) The electrolyte for a photoelectric conversion element described anyone of (e) to (h), wherein a primary average particle size of the carbonmaterial (C1) is from 0.5 to 120 μm.

(j) A photoelectric conversion element including a photoelectrodeincluding a transparent conductive film and a metal oxide semiconductorporous film;

a counterelectrode disposed opposite the photoelectrode; and

an electrolyte layer disposed between the photoelectrode and thecounterelectrode, wherein

the electrolyte layer is the electrolyte for a photoelectric conversionelement described in any of (a) to (i).

(k) A dye-sensitized solar cell including the photoelectrode describedin (j) carrying a photosensitized dye.

Effect of the Invention

As described below, the present invention is useful for providing anelectrolyte for a photoelectric conversion element, and a photoelectricconversion element and a dye-sensitized solar cell using theelectrolyte, wherein high energy conversion efficiency can be achievedwhile substantially not including iodine.

Additionally, the electrolyte for a photoelectric conversion element isextremely useful as high energy conversion efficiency can be achievedwithout using a p-type conducting polymer such as polyaniline.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an example of abasic configuration of a photoelectric conversion element of the presentinvention.

FIG. 2 is a drawing illustrating a basic configuration of a solar cellof the present invention used in the Working Examples and the like.

DETAILED DESCRIPTION

The present invention is explained in further detail below.

The electrolyte for a photoelectric conversion element of the presentinvention (hereinafter also referred to simply as “the electrolyte”)includes an ionic liquid (A) and a carbon material (B). The carbonmaterial (B) is a carbon material (B1) displaying a pH, measured by a pHmeasuring method specified in Japanese Industry Standard (JIS) Z8802, offrom 2 to 6 and/or a boron-modified acetylene black (B2). A content ofthe carbon material (B) is from 10 to 50 parts by mass per 100 parts bymass of the ionic liquid (A).

Additionally, the electrolyte of the present invention preferablyfurther includes a specific amount of a carbon material (C1) having aspecific surface area of from 1,000 to 3,500 m²/g and/or a specificamount of a carbon black (C2) having a nitrogen adsorption specificsurface area of not less than 90 m²/g as another carbon material (C),aside from the carbon material (B).

Next, each constituent of the electrolyte of the present invention willbe described in detail.

Ionic liquid (A)

The ionic liquid (A) for use in the electrolyte of the present inventionis not particularly limited, and any ionic liquid conventionally used inan electrolyte can be used.

For example, a quaternary ammonium salt, an imidazolium salt, apyridinium salt, a pyrrolidinium salt, a piperidinium salt, and the likedescribed in, “Ionic Liquids: The Front and Future of MaterialDevelopment”, Hiroyuki OHNO, CMC Publishing, 2003; “Functional Creationand Applications of Ionic Liquids”, Hiroyuki OHNO, NTS Publishing, 2004;and the like can be used.

The ionic liquid (A) includes cations and, as counterions thereto,anions.

Specific examples of preferred cations include the cations expressed byFormula (1) or (2) below.

In Formula (1), R¹ is a hydrocarbon group having from 1 to 20 carbonsthat may include a hetero atom, and may include a substituent having 1to 20 carbons that may include a hetero atom. R² and R³ are eachindependently a hydrogen atom or a hydrocarbon group having from 1 to 20carbon atoms, and may include a hetero atom. However, the R³ moiety isabsent if the nitrogen atom includes a double bond.

In formula (2), Q is a nitrogen, oxygen, phosphorus, or sulfur atom; andR⁴, R⁵, R⁶, and R⁷ are each independently a hydrogen atom or ahydrocarbon group having 1 to 8 carbons that may include a heteroatom.However, the R⁷ moiety is absent if Q is an oxygen or a sulfur atom.

The hydrocarbon group in Formula (1) having from 1 to 20 carbons andthat may include a hetero atom, R¹, preferably has a ring structurealong with the nitrogen atom (ammonium ion) in Formula (1).

Next, preferable examples of the substituent, having from 1 to 20carbons and that may include a hetero atom that R¹ in Formula (1) mayinclude, include alkyl groups having from 1 to 12 carbons (e.g. a methylgroup, an ethyl group, a propyl group, a butyl group, a pentyl group, ahexyl group, a heptyl group, an octyl group, and the like), alkoxygroups having from 1 to 12 carbons (e.g. a methoxy group, an ethoxygroup, an n-propoxy group, an iso-propoxy group, an n-butoxy group, atert-butoxy group, a sec-butoxy group, an n-pentoxy group, an n-hexoxygroup, a 1,2-dimethylbutoxy group, and the like), and alkylalkoxy groupshaving from 2 to 12 carbons (e.g. a methoxymethylene group (—CH₂OCH₃), amethoxyethylene group (—CH₂CH₂OCH₃), an n-propylene-iso-propoxy group(—CH₂CH₂CH₂OCH(CH₃)₂), a methylene-t-butoxy group (—CH₂—O—C(CH₃)₃, andthe like). Additionally, R¹ in Formula (1) may include two or more ofthese substituents.

Preferable specific examples of the hydrocarbon group, having from 1 to20 carbons and that may include a hetero atom that R² and R³ in Formula(1) may include, include alkyl groups having from 1 to 12 carbons (e.g.a methyl group, an ethyl group, a propyl group, a butyl group, a pentylgroup, a hexyl group, a heptyl group, an octyl group, and the like),alkoxy groups having from 1 to 12 carbons (e.g. a methoxy group, anethoxy group, an n-propoxy group, an iso-propoxy group, an n-butoxygroup, a tert-butoxy group, a sec-butoxy group, an n-pentoxy group, ann-hexoxy group, a 1,2-dimethylbutoxy group, and the like), alkylalkoxygroups having from 2 to 12 carbons (e.g. a methoxymethylene group(—CH₂OCH₃), a methoxyethylene group (—CH₂CH₂OCH₃), ann-propylene-iso-propoxy group (—CH₂CH₂CH₂OCH(CH₃)₂), amethylene-t-butoxy group (—CH₂—O—C(CH₃)₃, and the like), and the like.

Additionally, preferable specific examples of the hydrocarbon group,having from 1 to 8 carbons and that may include a hetero atom, R⁴, R⁵,R⁶, and R⁷ in Formula (2) include alkyl groups having from 1 to 8carbons (e.g. a methyl group, an ethyl group, a propyl group, a butylgroup, a pentyl group, a hexyl group, a heptyl group, an octyl group,and the like), alkoxy groups having from 1 to 8 carbons (e.g. a methoxygroup, an ethoxy group, an n-propoxy group, an iso-propoxy group, ann-butoxy group, a tert-butoxy group, a sec-butoxy group, an n-pentoxygroup, an n-hexoxy group, a 1,2-dimethylbutoxy group, and the like),alkylalkoxy groups having from 2 to 8 carbons (e.g. a methoxymethylenegroup (—CH₂OCH₃), a methoxyethylene group (—CH₂CH₂OCH₃), ann-propylene-iso-propoxy group (—CH₂CH₂CH₂OCH(CH₃)₂), amethylene-t-butoxy group (—CH₂—O—C(CH₃)₃, and the like), and the like.

Examples of the cations expressed by Formula (1) include imidazoliumions, pyridinium ions, pyrrolidinium ions, piperidinium ions, and thelike.

Specific examples of preferred cations include the cations expressed byany of Formulas (3) to (6) below.

Of these, the cations expressed by the following Formulas (3) and (5)are preferable because the photoelectric conversion efficiency of thephotoelectric conversion element used in the electrolyte of the presentinvention (hereinafter also referred to as the “photoelectric conversionelement of the present invention”) tends to be better.

In Formulas (3) to (6), R⁸ through R⁴⁰ are each independently ahydrocarbon group having from 1 to 20 carbons that may include anitrogen atom.

More specific examples include the following cations.

Examples of the cations of Formula (2) include organic cations such asammonium ions, sulfonium ions, phosphonium ions, and the like.

Specific examples of preferable cations are listed below.

Of these, aliphatic quarternary ammonium ions are preferable because thephotoelectric conversion efficiency of the photoelectric conversionelement of the present invention tends to be better.

On the other hand, specific examples of preferable anions included inthe ionic liquid (A) include I⁻,Br⁻, AlCl₄ ⁻, Al₂Cl₇ ⁻NO_(3,) ⁻, BF₄ ⁻,PF₆ ⁻, CH₃COO⁻, CF₃COO⁻, CF₃SO₃ ⁻, (CN)₄B⁻, SCN⁻, (CF₃SO₂)₂N⁻, (CN)₂N⁻,(CF₃SO₂)₃C⁻, (CN)₃C⁻, AsF₆ ⁻, SbF₆ ⁻, F(HF)_(n) ⁻, CF₃CF₂CF₂CF₂SO₃ ⁻,(CF₃CF₂SO₂)₂N⁻, CF₃CF₂CF₂COO⁻, and the like.

Of these, the anions are preferably bromine ions (Br⁻) or iodine ions(I⁻) and more preferably iodine ions (I⁻) because the photoelectricconversion efficiency of the photoelectric conversion element of thepresent invention tends to be better.

Examples of the ionic liquid (A) include combinations and the like ofthe cations and anions described above.

Of these, the ionic liquid (A) is preferably an ionic liquid includingimidazolium ions as the cations and iodine ions as the anions.

In the present invention, a synthesis method of the ionic liquid (A) isnot particularly limited, and various types of ionic liquids obtainedfrom combinations of the cations and the anions described above can besynthesized by a conventionally known method.

Synthesized products can be used such as 1-methyl-3-methyl imidazoliumiodide, 1-ethyl-3-methyl imidazolium iodide, 1-methyl-3-pentylimidazolium iodide, 1-hexyl-3-methyl imidazolium iodide,1-((2-methoxyethoxy)ethyl)-3-((2-methoxyethoxy)ethyl)imidazolium iodide,and the like, and also commercially available products can be used.Specific examples of commercially available products that can be usedinclude 1-methyl-3-propyl imidazolium iodide (manufactured by TokyoChemical Industry Co., Ltd.), 1-methyl-3-butyl imidazolium iodide(manufactured by Tokyo Chemical Industry Co., Ltd.),1-methyl-1-methyl-pyrrolidinium iodide (manufactured by Sigma-AldrichCo.), 1-ethyl-3-methylimidazolium tetracyanoborate (manufactured byMerck), 1-ethyl-3-methylimidazolium thiocyanate (manufactured by Merck),and the like.

In the present invention, a content of the ionic liquid (A) describedabove is preferably from 50 to 95 mass %, and more preferably from 65 to95 mass % of a total mass of the electrolyte of the present invention.If the content is within this range, the photoelectric conversionefficiency of the photoelectric conversion element of the presentinvention will be better.

Carbon Material (B)

The carbon material (B) having a content of from 10 to 50 parts by massper 100 parts by mass of the ionic liquid (A) is the carbon material(B1) displaying a pH of from 2 to 6 as measured in accordance with themeasuring procedure stipulated in JIS Z8802, and/or the boron-modifiedacetylene black (B2).

Here, when both the carbon material (B1) and the boron-modifiedacetylene black (B2) are included, the content (from 10 to 50 parts byweight) indicates a total content of both these components.

Carbon Material (B1)

The carbon material (B1) for use in the electrolyte of the presentinvention is a carbon material displaying a pH of from 2 to 6 asmeasured in accordance with the pH measurement procedures stipulated inJapanese Industrial Standard (JIS) Z8802.

The measurement of the pH is taken as described in detail below. First 5g of a carbon material sample is measured into a beaker. 50 mL of wateris added thereto and the water is heated to a boil. Then the heateddispersion is cooled to room temperature, and the carbon material isallowed to precipitate while at rest. Thereafter, the supernatant fluidis removed and the slurry is left behind. Then, an electrode of a glasselectrode pH meter is inserted into the slurry and a measurement istaken in accordance with the pH measurement procedures stipulated inJapanese Industrial Standard (JIS) Z8802-1984.

In the present invention, from 10 to 50 parts by mass of such a carbonmaterial (B1) per 100 parts by mass of the ionic liquid (A) areincluded. Therefore, high energy conversion efficiency can be achievedwhile substantially not including iodine.

While the specific reason is uncertain, it is thought that anions (e.g.carboxylate, phenolate and the like) derived from the acidic groups(phenolic hydroxy group, carboxy group, and the like described below)that exist as functional groups on the surface of the carbon materialare interacting with the cations of the ionic liquid (A) due to using acarbon material that is more acidic than conventional carbon blacks(acetylene black) and graphites. Moreover, it is thought that affinitygreater than that when using common carbon blacks (acetylene black) orgraphite is obtained due to the presence of the surface functionalgroups of the carbon material, and that this is a reason for the ease ofdispersion into the ionic liquid (A).

Additionally, in order to improve stability over time of the electrolytefor a photoelectric conversion element of the present invention andfurther enhance the photoelectric conversion efficiency of thephotoelectric conversion element of the present invention, a content ofthe carbon material (B1) is preferably from 15 to 50 parts by mass andmore preferably from 25 to 45 parts by mass per 100 parts by mass of theionic liquid (A).

Furthermore, in the present invention, a primary average particle sizeof the carbon material (B1) is preferably from 0.010 to 0.050 μm, andmore preferably from 0.010 to 0.035 μm. When the primary averageparticle size is within this range, the photoelectric conversionefficiency of the photoelectric conversion element of the presentinvention will be better.

Here, “primary average particle size” refers to a value taken by aconventional method for measuring the primary average particle size of acarbon material (e.g. furnace carbon black and the like), and, in thepresent invention, refers to an arithmetic average diameter of carbonblack particles calculated by observing using an electron microscope.

Examples of such a carbon material (B1) include an acidic carbon blackhaving an attached or bonded acidic group such as a phenolic hydroxygroup, a carboxy group, a quinone group, a lactone group, or the like; apigment carbon black; a color carbon black; and the like. Such a carbonmaterial (C3) may be used alone or in combinations of two or more.

Here, the carbon black, to which the hydroxy group is attached that isused for the acidic carbon black is not particularly limited, and acommonly used carbon black such as oil furnace black, gas furnace black,thermal black, acetylene black, channel black (gas black), and the likecan be used.

Additionally, examples of a method used to attach the hydroxy group orthe like include commonly performed ozone treatments, plasma treatments,liquid phase acidification treatments and the like, as well as methodssuch as that described in Japanese Unexamined Patent Application No.2004-238311.

Additionally, examples of the acidic carbon black that can be usedinclude a channel black (gas black) itself having multiple acidic groups(the phenolic hydroxy group, carboxy group, and the like describedabove) on a surface of the carbon black; and commercially availableproducts. Specific examples of commercially available products includethe following, all manufactured by Mitsubishi Chemical Corporation:#2200B, (pH: 3.5, primary average particle size: 0.018 μm), #1000 (pH:3.5, primary average particle size: 0.018 μm), #970 (pH: 3.5, primaryaverage particle size: 0.016 μm), MA77 (pH: 2.5, primary averageparticle size: 0.023 μm), MA7 (pH: 3, primary average particle size:0.024 μm), MA8 (pH: 3, primary average particle size: 0.024 μm), MA11(pH: 3.5, primary average particle size: 0.029 μm), MA100 (pH: 3.5,primary average particle size: 0.024 μm), MA100R (pH: 3.5, primaryaverage particle size: 0.024 μm), MA100S (pH: 3.5, primary averageparticle size: 0.024 μm), MA230 (pH: 3, primary average particle size:0.030 μm), MA200RB (pH: 3, primary average particle size: 0.030 μm), andMA14 (pH: 3, primary average particle size: 0.040 μm);

The following, all manufactured by Degussa Evonik Industries: SpecialBlack 6 (pH: 2.5, primary average particle size: 17 nm), Special Black 5(pH: 3.0, primary average particle size: 20 nm), Special Black 4 (pH:3.0, primary average particle size: 25 nm), Special Black 4A (pH: 3.0,primary average particle size: 25 nm), Special Black 550 (pH: 2.8,primary average particle size: 25 nm), Special Black 100 (pH: 3.3,primary average particle size: 50 nm), Special Black 250 (pH: 3.1,primary average particle size: 56 nm), Special Black 350 (pH: 3.5,primary average particle size: 31 nm), Printex 150T (pH: 4.0, primaryaverage particle size: 29 nm), Color Black FW1 (pH: 3.5, primary averageparticle size: 13 nm), Color Black FW18 (pH: 4.5, primary averageparticle size: 15 nm), Color Black FW285 (pH: 3.5, primary averageparticle size: 11 nm), Color Black 5170 (pH: 4.5, primary averageparticle size: 17 nm), Color Black 5160 (pH: 4.5, primary averageparticle size: 17 nm), Color Black FW200 (pH: 2.5, primary averageparticle size: 13 nm), Color Black FW2 (pH: 2.5, primary averageparticle size: 13 nm), and Color Black FW2V (pH: 2.5, primary averageparticle size: 13 nm);

The following, all manufactured by Tokai Carbon Co., Ltd.: TOKABLACK#8300F (pH: 5.0, primary average particle size: 16 nm) and TOKABLACK#8500F (pH: 5.5, primary average particle size: 14 nm); and the like.

One of these may be used alone, or two or more may be used in anycombination.

Among these, those having a pH of from 3 to 5.5 are preferable becausestability over time of the electrolyte for a photoelectric conversionelement of the present invention will be enhanced and the photoelectricconversion efficiency of the photoelectric conversion element of thepresent invention will be better.

Boron-Modified Acetylene Black (B2)

The boron-modified acetylene black (B2) for use in the electrolyte ofthe present invention is not particularly limited, and specific examplesthereof include boron-modified acetylene black (DENKA BLACK BMAB,manufactured by Denki Kagaku Kogyo K.K., specific surface area: 50 m²/g,primary average particle size: 35 nm, specific resistance: 1×10⁻² Ω·cm)and the like.

In the present invention, from 10 to 50 parts by mass of such aboron-modified acetylene black (B2) per 100 parts by mass of the ionicliquid (A) is included. Therefore, high energy conversion efficiency canbe achieved while substantially not including iodine.

While the specific reason is unclear, it is thought that, affinity tothe ionic liquid (A) increases and dispersion into the ionic liquid (A)is facilitated because the boron-modified acetylene black (B2) hasaffinity due to the introduction of boron, and that as a result theboron-modified acetylene black (B2), along with the ionic liquid (A),can contribute to the promotion of charge transfer. Note that with theboron-modified acetylene black (B2), boron is incorporated into thecarbon bonds and therefore has hole transport properties. Thus, due tothis as well, it is thought that the boron-modified acetylene black (B2)can contribute to charge transfer.

Additionally, in order to improve stability over time of the electrolytefor a photoelectric conversion element of the present invention andfurther enhance the photoelectric conversion efficiency of thephotoelectric conversion element of the present invention, a content ofthe boron-modified acetylene black (B2) is preferably from 15 to 50parts by mass and more preferably from 25 to 50 parts by mass per 100parts by mass of the ionic liquid (A).

Other Carbon Material (C)

The electrolyte of the present invention, in order to enhance thephotoelectric conversion efficiency of the photoelectric conversionelement of the present invention, preferably includes less than 10 partsby mass of a carbon material (C1) having a specific surface area of from1,000 to 3,500 m²/g and/or a carbon black (C2) having a nitrogenadsorption specific surface area not less than 90 m²/g per 100 parts bymass of the ionic liquid (A) as the other carbon material (C).

Here, when both the carbon material (C1) and the carbon black (C2) areincluded, the content (less than 10 parts by weight) indicates a totalcontent of both these components.

Carbon Material (C1)

The carbon material (C1) that can be used in the electrolyte of thepresent invention is a carbon material having a specific surface area offrom 1,000 to 3500 m²/g.

Here, “specific surface area” refers to a measurement taken using anitrogen adsorption BET method in accordance with the method stipulatedin JIS K1477.

Moreover, in the present invention, the specific surface area of thecarbon material (C1) is preferably from 1,100 to 3,200 m²/g, and morepreferably from 1,200 to 2,800 m²/g. When the specific surface area iswithin this range, filling of the metal oxide semiconductor porous filmwith the ionic liquid (A) and the satisfactory retaining of the ionicliquid (A) in the electrolyte are facilitated. Therefore, thephotoelectric conversion efficiency of the photoelectric conversionelement of the present invention is enhanced even more.

Furthermore, in the present invention, a primary average particle sizeof the carbon material (C1) is preferably from 0.5 to 120 μm, and morepreferably from 0.8 to 0.80 μm. When the primary average particle sizeis within this range, the photoelectric conversion efficiency of thephotoelectric conversion element of the present invention will bebetter.

Here, “primary average particle size” refers to a measurement takenaccording to a conventional method for measuring the primary averageparticle size of a carbon material (e.g. activated charcoal, furnacecarbon black, or the like), and, in the present invention, refers to a50% accumulated volume diameter (D50) measured by dispersing the carbonmaterial in a neutral detergent-containing aqueous solution and using alaser diffraction particle size distribution measurement device (e.g.the SALD2000J®, manufactured by Shimadzu Corporation).

Furthermore, in the present invention, an specific resistance of thecarbon material (C1) is preferably from 1×10⁻⁴ to 5×10² Ω·cm, morepreferably from 1×10⁻² to 1×10² Ω·cm, and even more preferably from5×10⁻² to 50 Ω·cm. When the specific resistance is within this range,surface graphitization does not progress, and therefore a carbonmaterial having excellent wettability with the ionic liquid (A)described above and high retention capacity of the ionic liquid (A) isobtained.

Specific examples of such a carbon material (C1) include activatedcharcoal (specific surface area: from 1,000 to 2,800 m²/g, primaryaverage particle size: from 0.5 to 120 μm, specific resistance: 1.0×10⁻¹Ω·cm); a boron-containing porous carbon material (specific surface area:from 1,000 to 2,000 m²/g, primary average particle size: from 0.5 to 100μm, specific resistance: 1×10⁻¹ Ω·cm); a nitrogen-containing porouscarbon material (specific surface area: from 1,000 to 2,000 m²/g,primary average particle size: from 0.5 to 100 μm, specific resistance:1×10⁻¹ Ω·cm); and the like. One of these may be used alone, or two ormore may be used in combination.

Of these, the activated charcoal is preferable because it is readilyacquirable.

The activated charcoal is not particularly limited, and conventionalactivated charcoal particles that are used in carbon electrodes and thelike can be used. Specific examples include activated charcoal particlesformed by activating coconut shell, wood dust, petroleum pitch, phenolicresins, and the like using water vapor, various chemicals, alkali, andthe like. One of these may be used or alone, or two or more may be usedin combination.

Carbon Black (C2)

The carbon black (C2) that can be used in the electrolyte of the presentinvention is not particularly limited so long as the nitrogen adsorptionspecific surface area is not less than 90 m²/g. Note that in the presentinvention, the carbon material (C1) is not included in the carbon black(C2).

Here, “nitrogen adsorption specific surface area” is an alternativecharacteristic of surface area that can be used in the adsorption ofcarbon black to the rubber molecules, and an amount of nitrogenadsorption to the surface of the carbon black is a measurement taken inaccordance with JIS K6217-7:2008 (Section 7: Rubber CompoundingIngredients—Determination Of Multipoint Nitrogen Surface Area (NSA) AndStatistical Thickness Surface Area (STSA)).

Additionally, in the present invention, a carbon black having a nitrogenadsorption specific surface area of from 90 to 200 m²/g is preferablyused and a carbon black having a nitrogen adsorption specific surfacearea of from 100 to 180 m²/g is more preferably used because thephotoelectric conversion efficiency of the photoelectric conversionelement of the present invention will be better.

Furthermore, in the present invention, the carbon black (C2) ispreferably a carbon black having a pH, measured in accordance with themeasuring method stipulated in JIS Z8802, from 7 to 13, and is morepreferably a carbon black with a pH from 7 to 11.

The measurement of the pH is taken as described below, the same as withthe carbon material (B1). First 5 g of a carbon black sample is measuredinto a beaker. 50 mL of water is added thereto and the water is heatedto boiling. Then the heated dispersion is cooled to room temperature,and the carbon black is allowed to precipitate while at rest.Thereafter, the supernatant fluid is removed and the slurry is leftbehind. Then, an electrode of a glass electrode pH meter is insertedinto the slurry and a measurement is taken in accordance with the pHmeasurement procedures stipulated in Japanese Industrial Standard (JIS)Z8802-1984.

A commercially available product can be used as such a carbon black(C2).

Specific examples include SAF (N134, nitrogen adsorption specificsurface area: 151 m²/g, pH: 7.3, manufactured by Cabot Japan K.K.), ISAF(N234, nitrogen adsorption specific surface area: 117 m²/g, pH: 7.5,manufactured by Cabot Japan K.K.), ISAF (N220, nitrogen adsorptionspecific surface area: 119 m²/g, pH: 7.5, manufactured by Cabot JapanK.K.), ISAF (N219, nitrogen adsorption specific surface area: 106 m²/g,pH: 7.5, manufactured by Tokai Carbon Co., Ltd.), HAF (N339, nitrogenadsorption specific surface area: 93 m²/g, pH: 7.5, manufactured byTokai Carbon Co., Ltd.), and the like.

In the present invention, the primary average particle size of thecarbon black (C2) is preferably from 5 to 30 nm, and more preferablyfrom 5 to 25 nm. When the primary average particle size is within thisrange, the photoelectric conversion efficiency of the photoelectricconversion element of the present invention will be better.

As described in relation to the carbon material (B1), here, “primaryaverage particle size” refers to an arithmetic average diameter ofcarbon black particles calculated by observing using an electronmicroscope.

By using such another carbon material (C) in the present invention, thephotoelectric conversion efficiency of the photoelectric conversionelement of the present invention will be better. This is thought to bebecause the other carbon material (C) forms an electrolyte thatsufficiently retains the ionic liquid (A) described above, and a metaloxide semiconductor porous film, which is described below, can besufficiently filled with the ionic liquid (A) from that electrolyte.

Additionally, the other carbon material (C) that has a large surfacearea has sponge-like functionality whereby the ionic liquid (A) can befilled and emptied. Therefore, it is thought that the formation oflocalized layers of the ionic liquid (A) (ionic liquid layers) formed ateach interface can be suppressed, specifically formation at an interfacebetween the electrolyte and the metal oxide semiconductor porous filmdescribed below, an interface between carbon particles, and an interfacebetween the electrolyte and the counterelectrode can be suppressed.Moreover, because a structure of the carbon black (C2) is advanced,electron conductivity is enhanced. For these reasons, it is thought thatthe open voltage of the photoelectric conversion element increased.Here, “open voltage” refers to the voltage between terminals whencurrent is not flowing into a power source terminal, and refers to avoltage measurement taken when current generated by light irradiationceases to flow against bias voltage (voltage flowing in an oppositedirection) when said bias voltage is applied to the electrode and thebias voltage is gradually increased (when a current value is zero).

Note that an ionic liquid must be present in the electrolyte for theelectrolyte to function as an electrolyte for a photoelectric conversionelement. However, for example, in the dye-sensitized photoelectricconversion element described in Patent Document 2, it was discoveredthat the ionic liquid forms an ionic liquid layer that has low chargetransport capacity between the interfaces described above. This ionicliquid layer may become a resistance component that leads to a decreasein photoelectric conversion efficiency.

Additionally, in the present invention, when the other carbon material(C) is included, a content thereof is less than 10 parts by mass andpreferably from 1 to 8 parts by mass per 100 parts by mass of the ionicliquid (A).

Furthermore, in order to enhance the filling of the metal oxidesemiconductor porous film with the ionic liquid (A), and also enhancethe sponge-like functionality and charge transfer described above, atotal content of the carbon material (B) and the other carbon material(C) is preferably from 10 to 50 parts by mass, more preferably from 15to 50 parts by mass, and even more preferably from 25 to 45 parts bymass per 100 parts by mass of the ionic liquid (A).

Additionally, for the same reasons, the content ratio [carbon material(B)/other carbon material (C)] of the carbon material (B) to the othercarbon material (C) is preferably from 99.9/0.1 to 60/40, morepreferably from 99/1 to 65/35, and even more preferably from 98/2 to70/30.

A redox couple can be added to the electrolyte of the present inventionin order to enhance the photoelectric conversion efficiency of thephotoelectric conversion element of the present invention.

Any conventional product commonly used for, or that can be used for,dye-sensitized solar cells may be used as the redox couple so long asthe object of the present invention is not impaired.

For example, metal complexes such as ferrocyanate-ferricyanate,ferrocene-ferricinium salt; sulfur compounds of a disulphide compoundand a mercapto compound; hydroquinone; quinone; and the like can beused. One of these may be used or alone, or two or more may be used incombination.

Additionally, an inorganic salt and/or an organic salt can be added tothe electrolyte of the present invention in order to enhance shortcurrent of the photoelectric conversion element of the presentinvention.

Examples of the inorganic salt and/or organic salt include alkalimetals, alkali earth metal salts, and the like, such as lithium iodide,sodium iodide, potassium iodide, magnesium iodide, calcium iodide,lithium trifluoroacetate, sodium trifluoroacetate, lithium thiocyanate,lithium tetrafluoroborate, lithium hexaphosphate, lithium perchlorate,lithium triflate, lithium bis(trifluoromethanesulphonyl)imide, and thelike. One of these may be used alone, or two or more may be used incombination.

An added amount of the inorganic salt and/or organic salt is notparticularly limited and may be a conventional amount so long as theobject of the present invention is not inhibited.

Additionally, a pyridine and/or a benzimidazole can be added to theelectrolyte of the present invention in order to enhance the openvoltage of the photoelectric conversion element of the presentinvention.

Specific examples include alkylpyridines such as methylpyridine,ethylpyridine, propylpyridine, butylpyridine, and the like;alkylimidazoles such as methylimidazole, ethylimidazole,propylimidazole, and the like; alkylbenzimidazoles such asmethylbenzimidazole ethylbenzimidazole, propylbenzimidazole, and thelike; and the like. One of these may be used or alone, or two or moremay be used in combination.

An added amount of the pyridine and/or the benzimidazole is notparticularly limited and can be a conventional amount.

An organic vehicle may be added to the electrolyte of the presentinvention, and specific examples thereof include carbonate esters suchas ethylene carbonate, propylene carbonate, and the like; ethers such asethylene glycol dialkyl ether, propylene glycol dialkyl ether, and thelike; alcohols such as ethylene glycol monoalkyl ether, propylene glycolmonoalkyl ether, and the like; polyhydric alcohols such as ethyleneglycol, propylene glycol, and the like; nitriles such as propionitrile,methoxypropionitrile, cyanoethyl ester, and the like; amides such asdimethylformamide, N-methylpyrrolidone, and the like; aprotic polarvehicles such as dimethyl sulfoxide, sulfolane, and the like; and thelike. One of these may be used or alone, or two or more may be used incombination.

An added amount of the organic vehicle is not particularly limited andcan be a conventional amount so long as the object of the presentinvention is not inhibited.

A manufacturing method of the electrolyte of the present invention isnot particularly limited and can, for example, be manufactured by mixingthe ionic liquid (A), the carbon material (B), and the optionallyincluded other carbon material (C), and then thoroughly mixing anduniformly dispersing (kneading) using a ball mill, sand mill, pigmentdisperser, grinder, ultrasonic disperser, homogenizer, planetary mixer,Hobart mixer, roll, kneader, or the like at room temperature or whilebeing heated (e.g. from 40 to 150° C.).

Here, as necessary, an organic solution (e.g. toluene or the like) canbe mixed in with the mixture described above and, after the mixing, theorganic solution may be removed using vacuum distillation.

Additionally, when including the other carbon material (C), the othercarbon material (C) may be finely pulverized beforehand using aconventional pulverizer such as a ball mill, jet mill, or the like inorder to thoroughly impregnate the ionic liquid (A) into the othercarbon material (C) when mixing. Moreover, for the same purpose, themixture of the ionic liquid (A), the carbon material (B), and theoptionally included other carbon material (C) may be subjected toreduced pressure treatment at room temperature or under heat (e.g. from40 to 150° C.).

In the present invention, when the other carbon material (C) isincluded, the electrolyte of the present invention is preferablymanufactured and prepared by obtaining a dispersion (e.g. a pastedispersion) by mixing the ionic liquid (A) and the carbon material (B)and, thereafter mixing in the other carbon material (C).

By preparing the electrolyte of the present invention according to sucha method, the photoelectric conversion efficiency of the photoelectricconversion element of the present invention will be better. It isthought that this is due to the carbon material (B) existing unevenly inthe ionic liquid (A); and the carbon material (B) interacting with thecations of the ionic liquid (A).

Next, the photoelectric conversion element and the dye-sensitized solarcell of the present invention will be described using FIG. 1. FIG. 1 isa schematic cross-sectional view illustrating an example of a basicconfiguration of a photoelectric conversion element of the presentinvention.

The photoelectric conversion element of the present invention includes aphotoelectrode having a transparent conductive film and a metal oxidesemiconductor porous film, a counterelectrode disposed so as to opposethe photoelectrode, and an electrolyte layer provided between thephotoelectrode and the counterelectrode.

Photoelectrode

As illustrated in FIG. 1, the photoelectrode is, for example,constituted by a transparent plate 1, a transparent conductive film 2,and an oxide semiconductor porous film 3.

Here, the transparent plate 1 preferably has excellent opticaltransparency, and specific examples include, in addition to glassplates, resin plates (films) such as polystyrene, polyethylene,polypropylene, polyethylene terephthalate, polyethylene naphthalate,polycarbonate, polyphenylene sulfide, cyclic olefin polymer, polyethersulfone, polysulfone, polyetherimide, polyarylate, triacetylcellulose,methyl polymethacrylate, and the like.

Additionally, specific examples of the transparent conductive film 2include conductive metal oxides such as tin oxide doped with antimony orfluorine, zinc oxide doped with aluminum or gallium, indium oxide dopedwith tin, and the like.

Moreover, a thickness of the transparent conductive film 2 is preferablyfrom about 0.01 to 1.0 μm.

Furthermore, the method for providing the transparent conductive film 2is not particularly limited, and examples thereof include coatingmethods, sputtering methods, vacuum deposition methods, spray pyrolysismethods, chemical vapor deposition (CVD) methods, sol-gel methods, andthe like.

Next, the oxide semiconductor porous film 3 is obtained by applying adispersion of oxide semiconductor particles on the transparentconductive film 2.

Specific examples of the oxide semiconductor particles include titaniumoxide, tin oxide, zinc oxide, tungstic oxide, zirconium oxide, hafniumoxide, strontium oxide, vanadium oxide, niobium oxide, and the like. Oneof these may be used alone, or two or more may be used in combination.

The dispersion is obtained by mixing the oxide semiconductor particlesand a carrier medium using a disperser such as a sand mill, bead mill,ball mill, three-roll mill, colloid mill, ultrasonic homogenizer,Henschel mixer, jet mill, or the like.

Additionally, the dispersion, after being obtained by mixing using thedisperser and immediately prior to use (application), is preferablysubjected to ultrasonic treatment using an ultrasonic homogenizer or thelike. By performing the ultrasonic treatment immediately prior to use,the photoelectric conversion efficiency of the photoelectric conversionelement of the present invention will be better. A reason for this isthought to be because the filling of the oxide semiconductor porousfilm, formed using the dispersion that has been subjected to ultrasonictreatment immediately prior to use, with the ionic liquid (A) isfacilitated.

Furthermore, acetyl acetone, hydrochloric acid, nitric acid,surfactants, chelating agents, and the like may be added to thedispersion in order to prevent the oxide semiconductor particles in thedispersion from re-aggregating; and a polymeric or cellulose thickeningagent such as polyethylene oxide, polyvinylalcohol, and the like may beadded to increase the viscosity of the dispersion.

Examples of commercially available products that can be used as thedispersion include titanium oxide pastes SP100 and SP200 (bothmanufactured by Showa Denko K.K.), titanium dioxide paste Ti-Nanoxide T(manufactured by Solaronix S.A.), Ti-Nanoxide D (manufactured bySolaronix S.A.), titania coating paste PECC01 (manufactured by PeccellTechnologies), titania particle paste PST-18NR (manufactured by NikkiChemical Co., Ltd.), titania particle paste PST400C (manufactured byNikki Chemical Co., Ltd.), and the like.

A conventional wet film forming method, for example, can be used as themethod for applying the dispersion on the transparent conductive film.

Specific examples of the wet film forming method include screen printingmethods, ink jet printing methods, roll coating methods, doctor blademethods, spincoating methods, spraying methods, and the like.

Additionally, after applying the dispersion on the transparentconductive film, a heat treatment, chemical treatment, plasma, or ozonetreatment is preferably performed in order to enhance electric contactbetween the particles, enhance adhesion with the transparent conductivefilm, and enhance film strength.

A temperature of the heat treatment is preferably from 40° C. to 700° C.and more preferably from 40° C. to 650° C. Additionally, a duration ofthe heat treatment is not particularly limited, but is normally fromabout 10 seconds to 24 hours.

Specific examples of the chemical treatment include chemical platingusing a titanium tetrachloride aqueous solution, chemisorption using acarboxylic acid derivative, electrochemical plating using a titaniumtrichloride aqueous solution, and the like.

Counterelectrode

As illustrated in FIG. 1, the counterelectrode is an electrode 5,disposed opposite a photoelectrode 4. For example, a metal plate, or aglass plate or a resin plate having a conductive film on a surfacethereof, can be used.

Examples of metals that can be used as the metal plate include platinum,gold, silver, copper, aluminum, indium, titanium, and the like. Examplesof resin plates that can be used include, in addition to the plate(film) exemplified by the transparent plate 1 that constitutes thephotoelectrode 4, common resin plates that are non-transparent or havelimited transparency.

Additionally, examples of the conductive film provided on the surfaceinclude conductive metal oxides and the like such as metals such asplatinum, gold, silver, copper, aluminum, indium, titanium, and thelike; carbon; tin oxide; tin oxides doped with antimony or fluorine;zinc oxide; zinc oxides doped with aluminum or gallium; indium oxidesdoped with tin; and the like. A thickness and a forming method of theconductive film are the same as for the transparent conductive film 2that constitutes the photoelectrode 4.

In the present invention, an electrode having a conductive polymericfilm formed on a plate or a conductive polymeric film electrode can beused as a counterelectrode 5.

Specific examples of the conductive polymer include polythiophene,polypyrrole, polyaniline, and the like.

Examples of a method for forming the conductive polymeric film on theplate include a method in which a conductive polymeric film from apolymeric dispersion is formed on a plate using a conventionally knownwet film forming method such as a dipping method or a spin coatingmethod.

Examples of products that can be used as the conductive polymericdispersion include a polyaniline dispersion described in JapaneseUnexamined Patent Application No. 2006-169291, commercially availableproducts such as a polythiophene derivative aqueous dispersion (BaytronP, manufactured by Bayer), Aquasave (manufactured by Mitsubishi Rayon,polyaniline derivative aqueous solution), and the like.

Additionally, when the plate is the conductive plate, in addition to themethod described above, the conductive polymeric film can also be formedon the plate via an electrolysis polymerization method. The conductivepolymeric film electrode can use a self-standing film wherein theconductive polymeric film formed on the electrode by the electrolysispolymerization method is peeled from the electrode, or a self-standingfilm formed using a casting method, a spin coating method, or the likethat is conventionally known as a wet film forming method for forming afilm from a conductive polymeric dispersion. Here, for convenience, amixture of a state in which conductive polymeric particles are dispersedthroughout the vehicle and a state in which conductive polymers aredissolved in the vehicle is referred to as the “conductive polymericdispersion.”

Electrolyte

As illustrated in FIG. 1, the electrolyte layer is an electrolyte layer6 that is provided between the photoelectrode 4 and the counterelectrode5. The electrolyte of the present invention described above is used inthe photoelectric conversion element of the present invention.

The photoelectric conversion element of the present invention canachieve high energy conversion efficiency while substantially notincluding iodine because the electrolyte of the present inventiondescribed above is used.

The dye-sensitized solar cell of the present invention is a type ofphotoelectric conversion element wherein the photoelectrode constitutingthe photoelectric conversion element of the present invention describedabove carries a photosensitized dye.

Here, the photosensitized dye is not particularly limited so long as itis a dye having absorption in the visible light spectrum and/or infraredlight spectrum, and a metal complex or an organic dye, or the like, canbe used.

Specific examples of the photosensitized dye that can be used includeruthenium complex dyes in which a ligand such as a bipyridine structure,a terbipyridine structure, or the like is coordinated, porphyrin-baseddyes, phthalocyanine-based dyes, cyanin-based dyes, melocyanine-baseddyes, xanthen-based dyes, and the like. A method for applying thephotosensitized dye is not particularly limited and, for example, can beapplied by dissolving the dye described above in, for example, water oran alcohol, and then immersing the oxide semiconductor porous film 3 inthe dye solution or coating the dye solution on the oxide semiconductorporous film 3.

EXAMPLES

The present invention will now be described in greater detail using thefollowing examples, but is in no way limited to these examples.

Working Examples 1 to 18 and Comparative Examples 1 to 8 Preparation ofthe Electrolyte

An ionic liquid and a carbon material, shown in Table 1 below, wereblended for 60 minutes in a mixing container according to thecompositions shown in Table 1 using a bead mill (Rocking RM02,manufactured by Seiwa Giken Co., Ltd.).

Specifically, in Working Example 1, 450 mg of the carbon material 1, 1.5g of the ionic liquid, 15 mL of toulene, and 10 g of zirconia beads(diameter: 3 mm) were placed in a 30 mL mixing container and blended for60 minutes using a bead mill (Rocking RM02, manufactured by Seiwa GikenCo., Ltd.).

Then, an electrolyte was obtained by distilling off the toluene in vacuofrom the dispersion after blending.

Note that for Working Examples 2 to 18 and Comparative Examples 1 to 8,with the exception of the composition mixing ratios being different, thesame preparation method as in Working Example 1 was used.

Fabrication of the Dye-Sensitized Solar Cell

A titanium oxide paste (Ti-Nanoxide D, manufactured by Solaronix) wascoated on transparent conductive glass (FTO glass, surface resistance:15 Ω/square, manufactured by Nippon Sheet Glass Co., Ltd.) and dried atroom temperature, and thereafter was sintered for 30 minutes at atemperature of 450° C. Thereby, a photoelectrode having a titanium oxideporous film formed on transparent conductive glass was fabricated.

The fabricated photoelectrode was then immersed for four hours in aruthenium complex dye(cis-(dithiocyanate)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)ruthenium(II) complex) (Ruthenium 535-bis TBA, manufactured bySolaronix) ethanol solution (concentration: 3×10⁻⁴ mol/L).

Thereafter, the product was washed using acetonitrile and dried in adark location under a stream of nitrogen. Thus a photoelectrode carryinga photosensitized dye in a titanium oxide electrode of a photoelectrodewas used as the photoelectrode.

The prepared electrolyte was applied on the photoelectrode carrying thephotosensitized dye, and this and a platinum counterelectrode formed byforming a platinum film having a thickness of about 100 nm on a surfaceof a transparent conductive glass plate using a sputtering method(indium oxide doped with tin on a conductive face, sheet resistance: 8Ω/square, manufactured by Nippon Sheet Glass Co., Ltd.) were aligned andthen bonded together using a clip, and thus the dye-sensitized solarcell was obtained.

The photoelectric conversion efficiency of the obtained dye-sensitizedsolar cell was measured and evaluated according to the method describedbelow. The results are shown in Table 1.

Photoelectric Conversion Efficiency

As illustrated in FIG. 2, a solar simulator is used as a light source,the photoelectrode side was irradiated with AM 1.5 artificial sunlightat a light intensity of 100 mW/cm², and the conversion efficiency wascalculated using a DC/AC measuring device (Digital SourceMeter 2400,manufactured by Keithley Instruments Inc.).

TABLE 1 Working Examples Comparative Example 1 2 3 4 5 6 7 8 1 2 3 4 5 67 Ionic liquid A1 100 100 100 100 100 100 100 100 100 100 100 100 100100 100 Carbon material B1-1 20 30 40 25 5 55 Carbon material Bl-2 30Carbon material Bl-3 30 Carbon material Bl-4 30 Carbon material B2-1 4010 5 55 Carbon black 1 30 Carbon black 2 30 Carbon black 3 30Photoelectric 4.9 5.2 5.1 4.6 4.8 4.9 5.2 5.3 3.1 3.9 2.8 3.7 3.8 2.03.0 conversion efficiency (%) Comparative Working Examples Example 9 1011 12 13 14 15 16 17 18 8 Ionic liquid A1 100 100 100 100 100 100 100100 100 100 100 Carbon material B1-1 10 20 20 20 20 20 25 25 Carbonmaterial B2-1 30 30 10 10 Carbon material C1-1 5 2 5 5 5 5 Carbonmaterial C2-1 8 8 8 Carbon material C2-2 8 Carbon material C2-3 8 Carbonblack 1 20 Carbon material B/Carbon 66.7/ 90.9/ 80/ 71.4/ 71.4/ 71.4/85.7/ 78.9/ 87.5/ 81.4/ material C 33.3 9.1 20 28.6 28.6 28.6 14.3 21.112.5 18.6 Carbon black 1/Carbon 80/20 material C Photoelectricconversion 5.0 5.2 5.3 5.4 5.3 5.2 5.5 5.5 5.7 5.7 3.9 efficiency (%)

The components listed in Table 1 are described below.

-   -   Ionic liquid A1: 1-methyl-3-propyl imidazolium iodide        (manufactured by Tokyo Chemical Industry Co., Ltd.)    -   Carbon material B1-1: Color Black FW1 (pH: 4.5, primary average        particle size: 13 nm, specific resistance: 5×10⁻¹ Ω·cm,        manufactured by Degussa)    -   Carbon material B1-2: Special Black 5 (pH: 3.0, primary average        particle size: 20 nm, specific resistance: 1.5 Ω·cm,        manufactured by Degussa)    -   Carbon material B1-3: TOKABLACK #8300F (pH: 5.0, primary average        particle size: 16 nm, specific resistance: 7×10⁻¹ Ω·cm,        manufactured by Tokai Carbon Co., Ltd.)    -   Carbon material B1-4: TOKABLACK #8500F (pH: 5.5, primary average        particle size: 14 nm, specific resistance: 5×10⁻¹ Ω·cm,        manufactured by Tokai Carbon Co., Ltd.)    -   Carbon material B2-1: Boron-modified acetylene black (DENKA        BLACK BMAB, specific surface area: 50 m²/g, primary average        particle size: 35 nm, specific resistance: 1×10⁻² Ω·cm,        manufactured by Denki Kagaku Kogyo K.K)    -   Carbon black 1: HAF (N326, nitrogen adsorption specific surface        area: 84 m²/g, pH: 7.5, manufactured by Tokai Carbon Co., Ltd.)    -   Carbon black 2: Printex X25 (nitrogen adsorption specific        surface area: 45 m²/g, pH: 9.0, primary average particle size:        56 nm, specific resistance: 1.5×10⁻¹ Ω·cm, manufactured by        Degussa)    -   Carbon black 3: Acetylene black (specific surface area: 68 m²/g,        pH: 9.5, primary average particle size: 35 nm, specific        resistance: 3×10⁻² Ω·cm, manufactured by Denki Kagaku Kogyo        K.K.)    -   Carbon material C1-1: Activated charcoal (NK261, specific        surface area: 2,300 m²/g, primary average particle size: 5 μm,        specific resistance: 1.5×10⁻¹ Ω·cm, manufactured by Kuraray        Chemical)    -   Carbon material C2-1: SAF (N134, nitrogen adsorption specific        surface area: 151 m²/g, pH: 7.3, average particle size: 19 nm,        manufactured by Cabot Japan K.K.)    -   Carbon material C2-2: ISAF (N234, nitrogen adsorption specific        surface area: 117 m²/g, pH: 7.5, average particle size: 23 nm,        manufactured by Cabot Japan K.K.)    -   Carbon material C2-3: HAF (N339, nitrogen adsorption specific        surface area: 93 m²/g, pH: 7.5, average particle size: 24 nm,        manufactured by Tokai Carbon Co., Ltd.)

As is clear from the results shown in Table 1, the electrolytes ofWorking Examples 1 to 8 that were prepared having a specific ratio ofthe ionic liquid (A) to the carbon material (B1) having a specified pHvalue and/or the boron-modified acetylene black (B2) achieved asufficiently high photoelectric conversion efficiency of from 4.6 to5.3% while substantially not including iodine. This unexpected resultshows that the electrolytes prepared in Working Examples 1 to 8 are notless than 1.5 times superior to the electrolyte prepared in ComparativeExample 7, in which acetylene black, an electrically conductivematerial, was used. Particularly, it was found that the electrolyte ofWorking Example 8, in which a combination of the carbon material (B1)and the boron-modified acetylene black (B2) was used, displayed anenhanced photoelectric conversion efficiency of 5.3%.

Additionally, it was found that the electrolytes of Working examples 9to 18, which further included the other carbon material (C) having aspecified specific surface area achieved a sufficiently highphotoelectric conversion efficiency of from 5.0 to 5.7% whilesubstantially not including iodine. This unexpected result shows thatthe electrolytes prepared in Working Examples 9 to 18 are not less than1.2 times superior to the electrolyte prepared in Comparative Example 8,in which a conventional carbon black was used at the same compositionratio in place of the other carbon material (C). Particularly, it wasfound that the electrolytes of Working Examples 17 and 18, in which theother carbon material (C) was compounded with the combination of thecarbon material (B1) and the boron-modified acetylene black (B2),displayed a sufficiently high photoelectric conversion efficiency of5.7%.

REFERENCE NUMERALS

-   1: Transparent plate-   2: Transparent conductive film-   3: Oxide semiconductor porous film-   4: Photoelectrode-   5: Counterelectrode-   6: Electrolyte layer-   11: Transparent plate-   12: Transparent conductive film (ITO, FTO)-   13: Metal oxide-   14: Electrolyte-   15: Platinum film-   16: Transparent conductive film (ITO, FTO)-   17: Plate-   18: Counterelectrode

1. An electrolyte for a photoelectric conversion element comprising anionic liquid (A) and a carbon material (B), wherein the carbon material(B) is a carbon material (B1) displaying a pH, measured by a pHmeasuring method specified in Japanese Industry Standard (JIS) Z8802, offrom 2 to 6 and/or a boron-modified acetylene black (B2), and a contentof the carbon material (B) is from 10 to 50 parts by mass per 100 partsby mass of the ionic liquid (A).
 2. The electrolyte for a photoelectricconversion element according to claim 1, wherein a primary averageparticle size of the carbon material (B1) is from 0.010 to 0.050 μm. 3.The electrolyte for a photoelectric conversion element according toclaim 1, wherein the ionic liquid (A) comprises a cation that isexpressed by the following Formula (1) or (2):

(in Formula (1), R¹ is a hydrocarbon group having from 1 to 20 carbonsthat may include a hetero atom, and may include a substituent having 1to 20 carbons that may include a hetero atom; R² and R³ are eachindependently a hydrogen atom or a hydrocarbon group having from 1 to 20carbon atoms, and may include a hetero atom. However, the R³ moiety isabsent if the nitrogen atom includes a double bond. In formula (2), Q isa nitrogen, oxygen, phosphorus, or sulfur atom; and R⁴, R⁵, R⁶, and R⁷are each independently a hydrogen atom or a hydrocarbon group having 1to 8 carbons that may include a heteroatom. However, the R⁷ moiety isabsent if Q is an oxygen or a sulfur atom).
 4. The electrolyte for aphotoelectric conversion element according to claim 3, wherein the ionicliquid (A) comprises iodine ions as anions.
 5. The electrolyte for aphotoelectric conversion element according to claim 1, furthercomprising less than 10 parts by mass of a carbon material (C1) having aspecific surface area of from 1,000 to 3,500 m²/g per 100 parts by massof the ionic liquid (A) as another carbon material (C), aside from thecarbon material (B).
 6. The electrolyte for a photoelectric conversionelement according to claim 1, further comprising less than 10 parts bymass of a carbon black (C2) having a nitrogen adsorption specificsurface area not less than 90 m²/g per 100 parts by mass of the ionicliquid (A) as the other carbon material (C), aside from the carbonmaterial (B).
 7. The electrolyte for a photoelectric conversion elementaccording to claim 5, wherein a total content of the carbon material (B)and the other carbon material (C) is from 10 to 50 parts by mass per 100parts by mass of the ionic liquid (A).
 8. The electrolyte for aphotoelectric conversion element according to claim 5, wherein a ratio[carbon material (B)/other carbon material (C)] of the carbon material(B) to the other carbon material (C) is from 99.9/0.1 to 60/40.
 9. Theelectrolyte for a photoelectric conversion element according to claim 5,wherein a primary average particle size of the carbon material (C1) isfrom 0.5 to 120 μm.
 10. A photoelectric conversion element comprising aphotoelectrode including a transparent conductive film and a metal oxidesemiconductor porous film; a counterelectrode disposed opposite thephotoelectrode; and an electrolyte layer disposed between thephotoelectrode and the counterelectrode, wherein the electrolyte layeris the electrolyte for a photoelectric conversion element according toclaim
 1. 11. A dye-sensitized solar cell comprising the photoelectrodeaccording to claim 10 carrying a photosensitized dye.
 12. Theelectrolyte for a photoelectric conversion element according to claim 6,wherein a total content of the carbon material (B) and the other carbonmaterial (C) is from 10 to 50 parts by mass per 100 parts by mass of theionic liquid (A).
 13. The electrolyte for a photoelectric conversionelement according to claim 6, wherein a ratio [carbon material (B)/othercarbon material (C)] of the carbon material (B) to the other carbonmaterial (C) is from 99.9/0.1 to 60/40.